Broadband optical via

ABSTRACT

A broadband optical via provides a low loss interconnection between waveguides in two vertically adjacent planar waveguiding layers. Two waveguides, one in each planar layer, evanescently interact over an interaction length, and substantially all of the power on one waveguide is transferred to the second waveguide. The relative detuning between waveguides is varied along the interaction region by tapering the width of one or both guides along the direction of propagation. The interaction strength is also varied by varying the physical separation between the two waveguides such that the interaction approaches zero near the two ends of the interaction length. The performance of the broadband optical via is fabrication tolerant, polarization tolerant, wavelength tolerant, and dimensionally tolerant.

REFERENCE TO RELATED APPLICATION

This is a continuation of patent application Ser. No. 11/473,613, filedJun. 23, 2006, which is a continuation of patent application Ser. No.10/776,706, filed Feb. 11, 2004, now U.S. Pat. No. 7,095,920, whichclaims priority under 35 U.S.C. Section 119 to provisional applicationSer. No. 60/446,612, filed Feb. 11, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and structure for interconnecting twointegrated optical waveguides that lie in different vertical planeswithin a multilayer optical circuit. More specifically, this inventionrelates to an optical via that transfers optical power from onewaveguide to a vertically adjacent waveguide, the power transfer beingfabrication tolerant, polarization tolerant, wavelength tolerant, anddimensionally tolerant.

2. Description of Related Art

Planar optical circuits are comprised of optical waveguides and devicesthat confine light to propagate primarily along a two-dimensional plane.Light is confined by an optical waveguide. The waveguide is comprised ofa core having a certain refractive index, and a cladding surrounding thecore on all sides. The cladding can be different on the top, sides, andbottom of the core, but at all locations it will have a refractive indexthat is lower than the refractive index of the core. The plane ofpropagation is parallel to the substrate on which the dielectric layerscomprising the waveguides are fabricated. It is possible to havenumerous planar layers each having waveguides vertically positioned orstacked one on top of the other. This increases integration densities.These stacked layers are often called multilayer optical circuits, orvertically integrated circuits.

Vertical integration is often used in silicon electronics both fordevice layers and for electrical interconnect (wiring) layers.Electrical signals can be routed from one layer to another layer by theuse of electrical vias, which are electrical connections in the verticaldirection. For an electrical via to work properly, it is sufficient tohave low resistance electrical connectivity between the layers throughthe optical via.

Optical vias, which transfer optical signals between adjacent planes,are not straight forward. Unlike electrical vias, it is not sufficientto have a continuity of optical core material for the photons to followbecause photons do not behave like electrons which simply follow a pathof least resistance. Photons can not traverse right angle turns withoutsignificant scattering loss.

A planar optical waveguide circuit comprised of two guiding layers isdepicted in FIGS. 1A to 1C. FIGS. 1A to 1C show three different crosssections of a portion of the optical waveguide circuit. FIG. 1A shows atop down schematic view pointing out the two waveguides (110, 111) thatare on different planar layers. FIG. 1B shows a lateral cross section ofthe structure, which in general comprises two waveguide cores (110,111), a substrate (116), a lower cladding (115), a buffer layer betweenthe two planar light guiding cores (113), a top cladding (112) andcladding around the two light guiding cores (114, 112). FIG. 1C shows alongitudinal cross section schematic of a portion of a two-layer opticalcircuit. Circuits similar to FIGS. 1A to 1C, having one or multiplelayers, are called “planar circuits” because propagation takes placemainly in a plane. The various waveguides in different vertical planarlayers do not interact except when they are close enough such that theoptical modes supported by the waveguides can interact with each other.This interaction range is usually limited to a distance smaller thanseveral optical wavelengths. The interaction is often called“evanescent” interaction because the evanescent fields of the modessupported by the waveguides interact. By extension, multilayer circuitsare similar to those in FIG. 1, having multiple guiding layerssurrounded by cladding layers, and adjacent guiding layers separated bybuffer layers.

An optical via is a structure that allows passage of an optical signalfrom one plane to another with low loss. One method to accomplish thisis the vertical directional coupler shown in FIGS. 2A to 2C. FIGS. 2A to2C are similar to FIGS. 1A to 1C, in that the structure described hastwo vertical waveguiding planar layers that are separated by a bufferlayer. FIGS. 2A to 2C describes a specific two layer optical circuitcalled a vertically coupled directional coupler. In this case twowaveguides in two planes are parallel to each other and directly aboveone another. The waveguides co-propagate together over some length. Theyare close enough so that the optical fields or modes in the twowaveguides can interact evanescently. If the waveguides have identicalpropagation constants, also called synchronous, they will exchange fullpower over a certain length called the beat length. The beat length is afunction of various geometrical waveguide parameters such as refractiveindex, geometric dimensions, and buffer layer thickness. The beat lengthis also a function of wavelength and polarization. If the length ofinteraction is longer than a beat length, power that has been coupledfrom one waveguide to another will couple back into the first waveguide,which is undesirable. If the length of interaction is shorter than abeat length, full power transfer will not be achieved. It is thereforeessential to correctly design the directional coupler to be exactly onebeat length long at the wavelength of interest. The directional couplertype of via is simple and short. Its drawbacks are that it is wavelengthand polarization sensitive and it is sensitive to all fabricationimperfections that cause the two waveguides to not be identical, suchas, for example, changes in refractive index, dimensions, or bufferlayer thickness.

U.S. Pat. No. 3,785,717 to M. Croset et al. describes a multilayeroptical circuit comprised of directional couplers, similar to FIGS. 2Ato 2C. The directional couplers are used to transfer optical powerbetween layers. The difficulty of directional couplers is that they arevery fabrication sensitive and also naturally wavelength dependent ornarrow band, and polarization dependent. The method specificallydescribed in U.S. Pat. No. 3,785,717 is also only limited todiffused-type waveguides, which are no longer used in state of the artoptical circuits.

U.S. Pat. No. 4,472,020 to V. L. Evanchuk et al. describes a method formaking monolithic circuits having waveguides on multiple layers. Themethod mainly pertains to fabrication methods to realize multilayerstructures. Via-like structures are discussed for the transfer ofoptical power between layers, and these structures amount to cornerreflectors or mirrors. In practice, integrated optic corner reflectorsand mirrors are very difficult to fabricate and are inherently verylossy. This loss can not be overcome easily. Further, corner reflectorsand mirrors need very high index contrast materials, or metallic layerswhich are inherently absorptive.

U.S. Pat. No. 6,236,786 to H. Aoki et al. describes a dual layer opticalcircuit where the two layers are connected by a through hole. As in U.S.Pat. No. 4,472,020 discussed above, corner reflectors or mirrors areused to transfer power between the two layers, using the through hole asa vertical light pipe to confine the light as it traverses from onelayer to the other. In practice, these structures are very lossy,especially for single mode waveguides, and required fabrication methodsthat are unconventional, and, therefore, not suitable for massproduction.

U.S. Pat. No. 3,663,194 to B. Greenstein et al. describes multilayeroptical circuits. Although the invention teaches methods to realizemultilayer circuits, these multilayers do not communicate with oneanother. Rather they are independent.

U.S. Pat. No. 4,070,516 to H. D. Kaiser describes a multilayer opticalcircuit and module. As in U.S. Pat. No. 4,472,020 discussed above,corner reflectors, mirrors, and right-angle bends are described for usein coupling the multilayers and changing the direction of the lightsignal in each layer. To date, such mirrors and corner reflectors havenot shown promise in practice, and, therefore, other means must beinvented. Further, light can not fundamentally be guided around aright-angle bend without loss and scattering. Photons, unlike electrons,can not be induced to follow right angle bends.

U.S. Pat. No. 6,650,817 to V. Murali describes a multi-level waveguide.The waveguides on multi-levels are interconnected by etching holes fromone layer to another and these holes filled with optically transparentand optically guiding material. This method uses unconventionalfabrication methods and is unsuitable for volume manufacturing. Further,as in earlier cited prior art above, light is forced to be guided aroundright-angle bends, which fundamentally are not low loss.

None of the prior art provides for efficient low loss power transferbetween optical waveguides in a multilayer optical circuit. Thisinvention discloses, for the first time, an efficient optical via thatprovides for low loss, fabrication tolerant, and broad wavelength usagepower transfer structure to connect optical waveguides on differentlayers of a multilayer optical circuit.

SUMMARY OF THE INVENTION

According to this disclosure, a broadband optical via provides a lowloss interconnection between at least two waveguides in spatiallydisposed, vertically adjacent planar waveguiding layers. The twowaveguides are sufficiently close to one another so as to evanescentlyinteract over an interaction length between them. Substantially all ofthe power propagating in one waveguide is transferred to the otherwaveguide. The relative detuning between waveguides is varied along theinteraction region by tapering the width of one or both guides along thedirection of propagation. The interaction strength is also varied byvarying the physical separation between the two waveguides such that theinteraction approaches zero near the two ends of their interactionlength. The performance of the broadband optical via is fabricationtolerant, polarization tolerant, wavelength tolerant, and dimensionallytolerant.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference symbols refer to like parts:

FIG. 1A shows a top down schematic view of a portion of multilayeroptical circuit.

FIG. 1B shows a lateral cross section schematic view of a portion of amultilayer optical circuit.

FIG. 1C shows a longitudinal cross section schematic view of a portionof a multilayer optical circuit.

FIG. 2A shows a top down schematic view of a vertically coupleddirectional coupler.

FIG. 2B shows a lateral cross section schematic view of a verticallycoupled directional coupler.

FIG. 2C shows a longitudinal cross section schematic view of avertically coupled directional coupler.

FIG. 3A shows a top down schematic view of a broadband optical via.

FIG. 3B shows a lateral cross section schematic view of a broadbandoptical via.

FIG. 3C shows a longitudinal cross section schematic view of a broadbandoptical via.

FIG. 4A depicts the waveguide-to-waveguide interaction strength alongthe length of the broadband optical via.

FIG. 4B depicts the waveguide-to-waveguide effective index detuningalong the length of the broadband optical via.

FIG. 5A depicts the robustness of the broadband optical via todeviations in the index difference between the two waveguides of theoptical via structure.

FIG. 5B depicts the robustness of the broadband optical via todeviations in waveguide width of one of the waveguides comprising theoptical via.

FIG. 5C depicts the robustness of the broadband optical via todeviations in the vertical waveguide 30 to waveguide buffer distance.

FIG. 5D depicts the robustness of the broadband optical via todeviations in the overall length of the optical via.

FIGS. 6A-6H describes fabrication steps for realizing multi-layeroptical circuits and broadband optical via.

FIG. 6A is step 1 relating to a substrate wafer and the deposition ofthe lower cladding layer.

FIG. 6B is step 2 relating to the deposition of the first core layer.

FIG. 6C is step 3 relating to applying of photoresist and etching thefirst core layer to form a ridge of core material.

FIG. 6D is step 4 relating to the deposition of a second cladding layer.

FIG. 6E is step 5 relating to the planarization of the upper claddinglayer.

FIG. 6F is step 6 relating to the deposition of the second core layer.

FIG. 6G is step 7 relating to applying of photoresist and etching thesecond core layer to form a ridge of core material.

FIG. 6H is step 8 relating to the deposition or a top cladding layer.

DETAILED DESCRIPTION OF DISCLOSURE EMBODIMENTS

We describe here the general principle of the broadband, fabricationinsensitive optical via. The via is based on the directional couplingbetween two waveguides situated on two vertically positioned layers, aspreviously described in conjunction with FIG. 2. However, in the presentcase the coupling strength between the two waveguides, and the relativedetuning in the propagations constants, or equivalently the effectiveindexes, are varied as a function of propagation distance in aprescribed manner. The resulting structure is longer than the simplebeat-length uniform directional coupler, but is more insensitive toexact length, wavelength, refractive index variations, geometricaldimensions and polarization.

FIGS. 3A to 3C are schematic views of the broadband optical viainvention. FIG. 3A shows a top-down schematic view of the broadbandoptical via. Two shaped waveguides on two different layers comprise theoptical via. FIG. 3B shows a lateral cross section schematic view of theoptical via. Waveguide 1 (310) is on planar layer 1 (347) and Waveguide2 (311) is on planar layer 2 (348). Layer 2 can be physically above orbelow layer 1. An incident optical signal from other parts of theoptical circuit is incident on the input (312) of Waveguide 1. A bufferlayer (341) separates the two waveguiding layers. The thickness of thisbuffer layer will be labeled “t” (345). The optical circuit includingthe optical via may be fabricated on a substrate (344) that additionallyincludes a lower cladding layer (343), a top cladding layer (340) thatsurrounds Waveguide 1, and cladding that surrounds Waveguide 2 (342). Byextension, there may be more than two waveguiding layers with bufferlayers separating the guiding layers and optical vias that interconnectthe various layers. FIG. 3C shows a longitudinal cross section schematicview of the broadband coupler. The function of the optical via is totransfer the entire signal in at the input of Waveguide 1 (312) to theoutput of Waveguide 2 (313), or vise versa, as shown by the arrow inFIG. 3C representing optical flow (346). Initially Waveguide 1 andWaveguide 2 are displaced laterally by a distance large enough such thatthey do not evanescently interact. Waveguide 1 is then curved by somegeneral input S-Bend (317 in FIG. 3A) such that it overlaps verticallywith Waveguide 2. The S-bend is not so abrupt that it causes bending orscattering loss. In referring to FIG. 3A, the input S-Bend (317) occursover a length L₁ (350). Waveguide 1 and Waveguide 2 then remain in closeproximity and parallel to each other over some propagation distance L₂(319). After the distance L₂ Waveguide 1 is again curved away fromWaveguide 2 by an output S-Bend (318), and the S-Bend occurs over alength L₁ (351) such that Waveguides 1 and 2 are separated sufficientlyfar away from each other that they do not interact. Waveguide 1 can beterminated at this point but does not need to be. Throughout the opticalvia, Waveguide 1 has a fixed width W₁ (314). Throughout the interactionregion, Waveguide 2 is linearly tapered in width. The narrow end of thetaper is towards the input side, while the wider end is towards theoutput side. Waveguide 2 is linearly tapered in such a way that thewaveguide starts with a width of W₂ (315) and ends with a width of W₃(316).

After the via section, Waveguide 2 can be again tapered to some otherdesired width, such as tapering to a width with dimension W₁. The lengthover which Waveguide 2 is tapered must be larger than L₂, but can beless than L₁+L₂+L₁. At the start of section L₂, Waveguide 2 must have awidth that is smaller than Waveguide 1. At the end of section L₂,Waveguide 2 must have a width that is larger than Waveguide 1.

Another way to describe the optical via is in terms of the couplingstrength between the two vertically coupled waveguides, and thedifference between the modal effective indexes, both as a function ofpropagation distance along the optical via. FIG. 4A shows the couplingor interaction strength between Waveguide 1 and Waveguide 2 of theoptical via of FIG. 3, as a function of distance along the optical via.It shows qualitatively how the coupling strength should vary along theoptical via. Referring to FIG. 4A, at the input end of the optical via(413) the coupling is small or negligible. Over the first S-Bendsection, Waveguides 1 and 2 approach each other and the couplingstrength increases (412). Over the section of length L₂, the waveguidesare positioned directly over one another and the coupling is strong andconstant (410). Over the second S-Bend region, the waveguides areseparated from each other and the coupling strength goes to zero (411).By the term “sufficiently far apart so as to not interact” we mean thatthe coupling strength is negligible for all practical purposes. FIG. 4Bshows the difference in modal effective indexes between Waveguides 1 and2 of the optical via of FIG. 3 as a function of propagation distance(420). The difference between the effective indexes is also termed the“detuning”. The notation N₁ and N₂ are used to describe the modaleffective indexes of Waveguide 1 and Waveguide 2, respectively. Near thecenter of the optical via, the effective indexes are identical (N₂−N₁=0)(421). We call this the synchronous condition. In the first portion ofthe optical via (422), the effective index of Waveguide 2 is smallerthan that of Waveguide 1, and hence N₂<N₁. In the second portion of theoptical via (423), the situation is reversed and the effective index ofWaveguide 2 is larger than that of Waveguide 1. The difference ineffective indexes varies smoothly along the optical via.

The description of the optical via in terms of position dependentcoupling strength and detuning is very useful because it describes theessence of the optical via without the need to specify the structure orgeometry. Several different structures and geometries can result in asimilar performing optical via. The structure and geometry areconsequences of trying to achieve the desired coupling and detuningprofiles. For instance, in order to achieve a smoothly varying detuning,Waveguide 2 is tapered, and to first order a linear taper leads to alinear detuning. It is likewise possible to taper Waveguide 1. Likewise,it is desirable to achieve a smoothly varying coupling strength, andthis is achieved by varying the separation between the two waveguides.

It should be noted that the optical via does not require preciselylinear tapers of the waveguides, or constant coupling strengththroughout the middle of the coupler. However, for descriptive purposes,and sometimes for fabrications purposes, linear tapers are simpler, butnot required.

The optical via is bidirectional. Referring to FIG. 3A, an opticalsignal incident at the input of Waveguide 1 (312) on Layer 1 will exitthe output of Waveguide 2 (313) on Layer 2. Reversing the direction, anoptical signal that is incident on the “output” Waveguide 2 on Layer 2,will be output at the “input” of Waveguide 1 in Layer 1.

It will be apparent to those skilled in the art, that the forgoingspecific geometry for the optical via is one of a number ofconfigurations that would lead to the same result. For instance, theS-bends on Waveguide 1 (317, 318) in FIG. 3 could be put on Waveguide 2.Similarly, the taper on Waveguide 2 can be placed on Waveguide 1, orsimultaneously on Waveguides 1 and 2, but with tapers in oppositedirections. Such modifications that lead to similarly functioningoptical vias, and are based on the same physics described in conjunctionwith FIG. 4 are to be considered equivalent embodiments within thespirit of this invention.

An example of a realized broadband optical coupler is now described. Abroadband optical via similar to the structure with respect to FIG. 3was analyzed and fabricated. Referring to FIG. 3, Waveguide 1 andWaveguide 2 had identical nominal core refractive indexes of 1.70. Thecladding index surrounding both cores on the sides, the top, the bottom,and the buffer layer in between was 1.45. The waveguides were both 1.5μm thick. The buffer layer had a thickness of t=350 nm. Waveguide 1 hada width of 1.1 μm and was constant along the optical via length. Notethat a vertically coupled directional coupler having two identicalwaveguides with the foregoing geometry would have a beat length of 54nm. A broadband optical via was designed with the following parameters,referring to FIG. 3 again: W₁=1.1 μm, W₂=0.5 μm, W₃=1.7 μm, L₁=150 μm,L₂=350 μm, and the guides are separated by 2 μm (center-center) beforeand after the optical via. FIG. 5 compares the optical via efficiencyfor (a) deviations in refractive index between the guides, (b)variations in waveguide widths, (c) variations in vertical couplingthickness, and (d) variations in coupling length L₂. Clearly, thebroadband optical via is more robust to fabrication deviations comparedto the simple beat-length directional coupler.

The foregoing broadband optical via device was experimentally fabricatedusing silicon oxynitride as the core material and silica as the claddingmaterial. The insertion losses of straight waveguides were compared tothose having two consecutive broadband optical vias (forming a bridge).It was found that the broadband optical vias introduced less than 0.1 dBof excess insertion loss, and the insertion loss did not vary over thewavelength from 1510 nm to 1620 nm. Hence, the optical via is broadband.

Method of Fabrication

One method of fabricating multi-core layer devices (also calledvertically coupled structures) is described in the sequence of stepsillustrated in FIG. 6. The three columns in FIG. 6 show the top downview, the lateral cross section view, and the longitudinal cross sectionview. The longitudinal cross section is along the center axis of thewaveguide. There are some novel fabrication steps, as well as some moreconventional steps. Details of conventional fabrication techniques maybe found in Hiroshi Nishihara, Masamitsu Haruna, Toshiaki, Suhara,“Optic Integrated Circuits” McGraw-Hill, 1985. Other methods offabricating multilayer circuits include B. E. Little et al. “VerticallyCoupled Glass Microring Resonator Channel Dropping Filters”, IEEEPhotonics Technology Letters, vol. 11, pp. 215-217, 1999, and P. P.Absil et al. “Vertically Coupled Microring Resonators Using PolymerWafer Bonding”, IEEE Photonics Technology Letters, vol. 13, pp. 49-51,2001.

Step 1, FIG. 6A: A substrate wafer is chosen as a carrier onto whichvarious dielectric layers will be deposited or grown. Common substratesinclude silicon, quartz, and Indium Phosphide. Often, a lower claddingmaterial is deposited or grown on the substrate for the purpose ofacting as a lower cladding or buffer to shield the optical mode from thesubstrate. A typical buffer layer on silicon is silicon dioxide. Thefirst step in FIG. 6 shows a cross section of a wafer with a lower clad.The lower clad is typically 3 μm to 15 μm thick.

Step 2, FIG. 6B: The core layer of the first waveguide is deposited as athin film. Common deposition techniques include chemical vapordeposition (CVD), sputtering, epitaxial growth, and spin on glasses orpolymers. Common materials that form the core are doped silica, silicon,silicon nitride, silicon oxynitride, compound glasses, spin on glass,optical polymers, and quaternary compounds such as indium galliumarsenide phosphide (InGaAsP). The amount of material deposited isdetermined by design requirements, and is well controlled in thedeposition steps. The second step in FIG. 6 shows a cross section of thechip with a thin film layer of core material used for the waveguide. Inthe present invention, the core is silicon oxynitride with n₁=1.70.

Step 3, FIG. 6C: Photoresist is spun onto the wafer, and the opticalcircuit layout is photographically patterned into the photoresist. Thepattern comes from a design mask that is used as a photographic plate.The design includes any tapering or bends within the 10 waveguides. Thephotoresist pattern is transferred into the core layer by standardetching techniques that use the photoresist as a mask. Etchingtechniques include chemical wet etching or reactive ion etching. Afterpattern transfer, the remaining photoresist is striped away, leaving aridge or strip of core material that forms the first core of the opticalcircuit. The third step in FIG. 6 shows a cross section of the chipafter the thin film layer has been etched, and the photoresist has beenstriped off.

Step 4, FIG. 6D: An intermediate cladding material is deposited or grownon the surface of the wafer covering the first waveguide to a thicknessgreater than the core depth. This material can be any described in theprevious steps, but will have an index that is lower than that of thefirst core layer (that is n_(i)<n₁). Because of the topography, the topsurface of this layer may not be planar. For example, there may be aridge over the waveguide, as depicted in step 4 of FIG. 6. Step 4 inFIG. 6 shows a cross section of the chip after top cladding material hasbeen deposited over the wafer.

Step 5, FIG. 6E: It is desired to have the first waveguide encapsulatedin the cladding material on all sides, except for the top surface of thewaveguide. Therefore, the cladding that was deposited in step 4 must beremoved down to a thickness of the first waveguide height, and must beplanarized to give a flat surface across the chip or wafer. The topcladding can be planarized by well known techniques such as etch backand/or polishing. A planar buffer layer is then regrown. The fifth stepin FIG. 6 shows a cross section of the chip after planarization down tothe top surface of the first waveguide.

Step 6, FIG. 6F: Similar to step 2, material is deposited or grown overthe surface of the wafer. This material will comprise the second corelayer, and will have a refractive index of n₂. Any of the foregoingmaterials discussed in the previous steps may comprise this second corelayer. In general, the index of this material will be close to therefractive index of the first waveguide. Step 6 of FIG. 6 shows a crosssection of the chip after the second core layer is deposited.

Step 7, FIG. 6G: Similar to step 3, photoresist is spun onto the wafer,and the second core layer is photographically patterned. The pattern istransferred to the second core layer by etching the material. Thephotoresist is striped away, and the result is ridges that are now thesecond core layer, and which lie directly above the first core layer.Any tapering within the mode transformer section for the second core ison the lithographic mask. Step 7 in FIG. 6 shows a cross section of thechip after material for the second core layer has been etched and thephotoresist has been striped.

Step 8, FIG. 6H: Finally, cladding material is deposited over the entirewafer. Step 8 in FIG. 6 shows a cross section of the wafer with a topcladding layer deposited over the entire surface. The surface of the toplayer may or may not be planar. If a planar surface is desired,planarization techniques similar to those described in Step 5 may beused. Steps 1 to 7 can be repeated to add additional core layers.

While the preferred embodiments have been described, it will be apparentto those skilled in the art that various modifications may be made tothe embodiments without departing from the spirit of the presentinvention and such modifications are within the scope of this invention.

1. An optical via formed between waveguides formed in a multi-layerwaveguide device comprising: a first waveguide having a first refractiveindex for supporting a first optical mode; a second waveguide having asecond refractive index for supporting a second optical mode; the firstand second modes of the waveguides evanescently interacting over aninteraction length between the two waveguides; the magnitude of themode-to-mode evanescent interaction diminished in interaction strengthin the second waveguide at a first end of the interaction length anddiminished in interaction strength in the first waveguide at a secondend of the interaction length to substantially complete optical powertransfer of propagating light from the first waveguide to the secondwaveguide over the course of the interaction length.
 2. The optical viaof claim 1 wherein a width of one of the first and second waveguides isfixed over the interaction length and a width of the other of the firstand second waveguides varies monotonically in width over the interactionlength.
 3. The optical via of claim 2 wherein the other waveguidemonotonically increases in width over the interaction length.
 4. Theoptical via of claim 3 wherein the other waveguide is the secondwaveguide.
 5. The optical via of claim 3 wherein the monotonical widthincrease is in the direction of propagating light in the firstwaveguide.